1. Field of the Invention
The present invention relates to an insulated gate semiconductor device, and particularly to an insulated gate semiconductor device in which an insulated gate semiconductor element to be used for current control for a luminous tube is prevented from being broken by an increase in the rate of voltage change.
2. Description of the Related Art
Insulated gate bipolar transistors (hereinafter referred to as IGBTs) have been used as switching elements for performing current control and the like for light-emitting devices (flashes) for use in digital still cameras and mobile phone's camera functions (e.g., see Japanese Patent Application Publication No. 2005-302380).
With reference to FIG. 9, one example of a conventional light-emitting device will be described. FIG. 9 is a circuit diagram showing the entire configuration of the light-emitting device. A description will be made by taking as an example the case where a xenon discharge tube is used as a luminous tube.
The principal operation of the circuit will be described with reference to FIG. 9. A step-up transformer 36 raises the voltage of a power-source battery 38 to a predetermined voltage, and charge is stored in a main capacitor 35. When a switching element 60 is turned on by a gate drive circuit 37, a trigger voltage is applied from a trigger circuit 30 to a xenon discharge tube as a luminous tube 40. Upon receipt of the trigger voltage, the luminous tube 40 starts light emission. Turning off the switching element 60 with predetermined timing stops the luminous tube 40 from emitting light.
The switching element 60 is principally intended to control the stopping of light emission with high accuracy, and is an IGBT with high power and excellent response characteristics. A gate G of the switching element (IGBT) 60 is connected to the gate drive circuit 37 through a gate resistor Rg. The turning on/off of the IGBT 60 is controlled based on a signal from the gate drive circuit 37.
FIG. 10 is a plan view showing part of a chip of the IGBT 60. The IGBT 60 (the chip thereof) is provided with a gate-to-emitter protection diode 60d and an active area (two-dot chain lines) 60e where transistor cells are disposed. The active area 60e includes, for example, a trench 63 having inner walls covered with a gate insulating film (not shown), a gate electrode 64 buried in the trench 63, and emitter regions 66 provided adjacent to the trench 63. The regions surrounded by the trench 63 serve as transistor cells. On the active area 60e, an emitter electrode 67 is provided with an insulating film (not shown) interposed therebetween. The emitter electrode 67 is in contact with the emitter regions 66 through emitter contact regions 65. The protection diode 60d is disposed outside the active area 60e, e.g., in a chip corner portion. The gate electrode 64 of the active area 60e is connected to a gate pad portion 69 through a gate interconnection portion 68.
FIG. 11 is a view showing the relationship between each of collector-emitter voltage VCE, collector current IC, and gate voltage VG and turn-off loss during the turn-on and turn-off of the IGBT 60.
A turn-on interval (rise time tr) is the interval (period of time) taken by the collector-emitter voltage VCE to decrease from 90% to 10%, and a turn-off interval (fall time tf) is the interval (period of time) taken by the collector current IC to decrease from 90% to 10%. Loss (current×voltage) during the turn-on interval and loss during the turn-off interval are called turn-on loss and turn-off loss, respectively. In FIG. 11, the slope of the collector-emitter voltage VCE in the turn-off interval is dv/dt. The hatched portion in FIG. 11 represents the value of the turn-off loss.
The gate resistor Rg is externally connected to the chip of the IGBT 60 (FIG. 9). The rate (hereinafter referred to as dv/dt) of change of the collector-emitter voltage VCE during the turn-off of the IGBT 60 is adjusted using the resistance of the gate resistor Rg. Further, the time (rise time tr) taken by the IGBT 60 to be turned on is determined by the value of the gate resistor Rg.
In IGBTs, when the value of dv/dt during turn-off is large, transistor cells operate unevenly to cause the concentration of heat on some of the cells due to characteristics of IGBTs. This may result in a breakdown of an IGBT (such breakdown is hereinafter referred to as dv/dt breakdown). Accordingly, in general, a circuit configuration is employed in which the gate resistor Rg is connected to the gate G of the IGBT 60 as in FIG. 9. The value of dv/dt can be reduced by increasing the resistance of the gate resistor Rg. Thus, a dv/dt breakdown of the IGBT 60 can be prevented.
However, if the resistance of the gate resistor Rg is increased more than needed, i.e., if the value of dv/dt during turn-off is too small, the area of the hatched portion increases accordingly, and therefore turn-off loss increases (FIG. 11). In the case where turn-off loss is too large, the IGBT 60 is broken by the heat caused by the turn-off loss (breakdown due to turn-off loss is hereinafter referred to as thermal breakdown). Thus, there is a trade-off between dv/dt breakdown and thermal breakdown, and the resistance of the gate resistor Rg needs to be adjusted to an optimum value.
FIGS. 12A to 12C are diagrams showing other examples of a circuit including the switching element 60 and gate resistors connected thereto.
FIG. 9 shows a configuration in which one gate resistor Rg is connected to the gate G of the IGBT 60. Both a gate charge current during turn-on and a gate discharge current during turn-off pass through the same gate resistor Rg. Accordingly, a characteristic (e.g., dv/dt) during turn-off and a characteristic (e.g., rise time) during turn-on cannot be controlled separately.
On the other hand, each of the circuits in FIGS. 12A to 12C has a configuration in which a gate resistor Rgon and a rectifier diode 70 connected in series are connected to the gate G of the IGBT 60 and in which the rectifier diode 70 and a gate resistor Rgoff are connected in parallel. In this configuration, a gate charge current flows into the gate G of the IGBT 60 through the resistor Rgon and the rectifier diode 70 during turn-on, and a gate discharge current flows through the resistor Rgoff (and the resistor Rgon) during turn-off. Since the resistors Rgon and Rgoff can be set separately, a characteristic during turn-on and a characteristic during turn-off can be controlled independently.
As described above, in an IGBT, it is desirable that a characteristic during turn-on and a characteristic during turn-off can be controlled independently. A configuration such as shown in FIGS. 12A to 12C is preferable in which the gate resistors Rgon and Rgoff and the rectifier diode 70 are externally connected to the chip of the IGBT 60 to cause a gate charge current during turn-on and a gate discharge current during turn-off to flow through the different gate resistors Rgon and Rgoff, respectively.
In particular, for the purpose of preventing a dv/dt breakdown of an IGBT used as a switching element for current control for a light-emitting device, it is important to appropriately select the gate resistor Rgoff having a resistance in a range which allows the IGBT to operate safely and desired characteristics to be obtained.
Accordingly, when a chip of an IGBT or a package product in which an IGBT chip is sealed with resin or the like is supplied to a user, it is recommended to use the IGBT within its rating in which the operation of the IGBT is guaranteed (e.g., to connect the gate resistor Rgoff having such a resistance that dv/dt is 400 V/μs or less).
However, when the chip of the IGBT 60 is configured to be externally connected to the gate resistor Rgoff as shown in FIGS. 12A to 12C (or the gate resistor Rg in FIG. 9 in the same manner), a user may connect the chip of the IGBT 60 to a gate resistor Rgoff having a resistance out of the dv/dt rating. This may cause a problem of a dv/dt breakdown of the IGBT 60.